Subarrays That Sum to K

Write count_subarrays(nums, k) that counts the subarrays of nums whose items add up to exactly k. A subarray is a non-empty stretch of neighbouring items, such as nums[2:5]. Two subarrays at different positions count separately, even if they hold the same values.

The numbers can be negative or zero.

Examples

count_subarrays([1, 1, 1], 2)   # returns 2
count_subarrays([1, 2, 3], 3)   # returns 2
count_subarrays([3, -3, 3], 0)  # returns 2
count_subarrays([0, 0, 0], 0)   # returns 6

In the second example the two subarrays are [1, 2] and [3].

Constraints

• 0 <= len(nums) <= 20,000
• -1,000 <= nums[i] <= 1,000
• k is between -10**7 and 10**7